After beating the elite four and obtaining the rainbow ticket, heracross can be found in the forest north-east of six island.
heracros has no evolution
No, you cannot find cyndaquil in LeafGreen.
LeafGreen version does not have Growlithe.
There aren't growlithe in LeafGreen. FireRed has growlithe, LeafGreen has vulpix.
That item doesn't exist in leafgreen.
you cant go underwater in leafgreen!!
Piplup does not exist in Pokemon LeafGreen or FireRed.
Psyduck is in firered not leafgreen you can find it using a good or super rod in the safari zone.
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you cant
Nowhere.
there is no such Pokemon